NEW YORK (CNN) -- Reconstruction of the World Trade Center site is over budget and won't be finished by the 10th anniversary of the September 11, 2001, terrorist attacks that destroyed the twin towers, the owners reported Monday.

"While significant progress has been made, the schedule and cost estimates of the rebuilding effort that have been communicated to the public are not realistic," Christopher Ward, executive director of the Port Authority of New York and New Jersey, wrote in a letter to New York Gov. David Paterson.

Paterson asked for a status update on the construction efforts earlier this month. The Port Authority, which owns the site, released its findings at a board meeting Monday.

"Progress has been made. Ground zero has gone from a standstill to a busy construction site, but the rebuilding effort is not where it should be and it is not where we promised it would be," Paterson said in a written statement after the announcement.

The complex was destroyed when al Qaeda operatives flew hijacked jetliners into the landmark twin towers on September 11, 2001, killing more than 2,700 people. The Port Authority listed 15 separate issues, including security and policing plans and contracting strategies for rebuilding the subway stations that sat beneath the center, that it said are affecting the reconstruction efforts.

"The questions surrounding the World Trade Center rebuilding is not an 'if' all of the projects will be built, rather when and for how much," Ward wrote.

While Ward did say progress was being made, he said "a new way" of doing business must be in place to ensure the rebuilding moves forward. He calls for new efforts to make sure everyone with a stake in the project works together.

The Port Authority said the second phase of its assessment will be completed by the end of September and hopes to have worked through unresolved issues and have accurate final schedules and cost estimates to release by then.